1. Field of the Invention
The present invention relates to a zoom lens system, an imaging device and a camera. In particular, the present invention relates to a zoom lens system that has a high resolution, high capability of compensating curvature of field, a high zoom ratio of, for example, 3 or greater, a reduced weight, and a reduced overall optical length at the time of non-use; and an imaging device and a camera that employ the zoom lens system so as to have a reduced thickness and excellent portability as well as high performance.
2. Description of the Background Art
In the prior art, a large number of optical instruments have been developed that form an image of a photographic object onto an image sensor through a lens and then acquire the object image as an image. Recently, products such as digital still cameras and digital video cameras are spreading. Then, with the increase in the number of users, desire on these products is also growing. Among various types of these products, optical instruments having a zoom ratio of approximately 3 are relatively small and still have an optical zoom function. Thus, these types are spreading remarkably widely as digital cameras of compact type or stylish type.
In the digital cameras of compact type, for the purpose of the property of easy carrying, further size reduction of the instruments is desired. In order to achieve the further size reduction of the digital cameras, the lens arrangement need be adopted such that the overall optical length (the distance measured from the top of the most object side lens surface of the entire lens system to the image surface) at the time of non-use should be reduced while lens elements that extend out relative to the main body by means of a multi-stage lens barrel at the time of use could be accommodated into the main body. Further, in the digital cameras to spread widely, cost reduction is also desired.
Meanwhile, as zoom lens systems suitable for digital still cameras of compact type, a large number of zoom lens systems of three-unit construction have been proposed that, for example, in order, form the object side to the image side, comprise a first lens unit having negative optical power, a second lens unit having positive optical power, and a third lens unit having positive optical power.
In such a zoom lens system of three-unit construction, in zooming (magnification change) from a wide-angle limit to a telephoto limit, the air space between the first lens unit and the second lens unit decreases monotonically, while the air space between the second lens unit and the third lens unit varies, and while the third lens unit is fixed or moved.
Focus adjustment in the zoom lens system of three-unit construction is performed by moving the first lens unit or the third lens unit in the optical axis direction. In particular, from the perspective of size reduction of the entire optical instrument, in many cases, the focus adjustment is performed using the third lens unit which is less heavy, so that focusing onto the photographic object is achieved ranging from the infinity to a short distance. In contrast, when the focus adjustment is performed using the first lens unit, the first lens unit is larger than the third lens unit and hence requires a large size motor. This causes a tendency of size increase in the entire optical instrument.
The third lens unit having positive optical power has the effects of compensating curvature of field and bringing into a telecentric state the incident light onto the imaging surface. Further, in many cases, the third lens unit is composed of one or two lens elements having a small outer diameter, and hence can be driven at a high speed using a small size motor. Thus, when the third lens unit is adopted as a lens unit for focus adjustment, an optical instrument is realized that has a reduced size and permits rapid focusing.
The first lens unit and the second lens unit move in parallel to the optical axis along a cam groove formed in a cylindrical cam. In the cam groove, a groove for zooming and a groove for the time of non-use are connected to each other. The groove for the time of non-use reduces the interval between the lens units and moves all the three lens units to the image sensor side. This configuration reduces the overall optical length at the time of non-use. In this case, if the thickness of each lens unit could be reduced, the overall optical length at the time of non-use would be reduced further.
As such, in the prior art, design has been performed such that the zoom lens system should have the above-mentioned configuration where the size is reduced in the part relevant to focus adjustment and in the entire lens system at the time of non-use, so that the overall optical length of the digital still camera has been reduced and so has been the cost.
For example, Japanese Laid-Open Patent Publication No. 2006-10895 discloses a three-unit zoom lens, in order from the object side to the image side, comprising: a first lens unit having negative optical power which is, composed of a negative power lens and a positive power lens; a second lens unit having positive optical power which is composed of a cemented lens consisting of a convex lens and a concave lens and a single lens; and a third lens unit having positive optical power. In this three-unit zoom lens, an aspheric surface is employed in the first lens unit and in the second lens unit, while a synthetic resin lens is employed as the single lens of the second lens unit and in the third lens unit. By virtue of this, in a state that aberration is compensated satisfactorily, the individual lens thicknesses are reduced so that size reduction and cost reduction are achieved in the optical system.
Further, for example, in a zoom lens disclosed in Japanese Laid-Open Patent Publication No. 2004-144947, two positive lenses are provided on the image side relative to the second unit so that the optical power per lens is reduced. Then, these lenses having relatively low optical power are composed of plastic lenses. In general, in plastic lenses, the image surface position and the like easily fluctuate owing to environmental variation. However, in the zoom lens disclosed in Japanese Laid-Open Patent Publication No. 2004-144947 where such plastic lenses having relatively low optical power are employed, influence of environmental variation is rather small, and hence view angle enhancement in the shooting view angle and weight reduction in the lens system are achieved simultaneously.
Further, for example, in a zoom lens disclosed in Japanese Laid-Open Patent Publication No. 2002-372667, an aspheric surface is employed in the most image side lens of the second lens unit, so that astigmatism is compensated. Further, within the second lens unit, at least a lens arrangement of positive, positive and negative in order from the object side is included. Further, an aspheric plastic lens is arranged on the most image side. By virtue of this, spherical aberration and coma aberration are compensated, while the Petzval sum is reduced so that curvature of field is reduced. As such, in the zoom lens disclosed in Japanese Laid-Open Patent Publication No. 2002-372667, aberration generated in the second lens unit is reduced so that aberration fluctuation during zooming is reduced. Further, the optical power in one lens unit is divided so that the optical power per lens is reduced. This reduces spherical aberration and coma aberration generated in the positive lenses. Moreover, since a plastic lens is employed, injection molding is adopted and permits easy aspheric surface formation, manufacturing error reduction, and weight reduction in the entire lens.
Further, for example, in a variable magnification optical system disclosed in Japanese Laid-Open Patent Publication No. 2006-11096, at least one aspheric surface is employed in the first lens unit so as to compensate astigmatism and distortion generated when the negative optical power of the first lens unit is increased. This permits size reduction. Further, in the first lens unit, a lower limit is placed on the absolute value of the refractive index difference of the lenses, so that the Petzval sum is reduced. Then, astigmatism and curvature of field are compensated.
In the configuration of the variable magnification optical system disclosed in Japanese Laid-Open Patent Publication No. 2006-11096, a negative lead is adopted in the first lens unit on the most object side such that that the light beam incident at a large angle from the object side to the lens surface should be refracted greatly, so that size reduction is achieved. Further, a lower limit is placed on the maximum value of refractive index in the lenses within the first lens unit. This reduces the radii of curvature of the lenses, and hence suppresses an increase in the generated aberration. Moreover, a limit is placed on the optical power of the second lens unit, so that an increase is suppressed in decentration error sensitivity and manufacturing difficulty generated in the second lens unit in a state that the variable magnification ratio and compactness are maintained satisfactorily.
Further, for example, in a zoom lens disclosed in Japanese Laid-Open Patent Publication No. 2006-194974, a two-lens construction is adopted in the first lens unit so that the size in the retracted state is reduced and hence overall thickness reduction is achieved. Further, various kinds of aberration generated in the first lens unit in association with the thickness reduction are suppressed by increasing the refractive indices of the individual lenses and increasing the curvatures of the lens surfaces. In particular, axial chromatic aberration and magnification chromatic aberration are suppressed by placing upper and lower limits on the refractive index difference between the second lens and the first lens in the first lens unit and on the Abbe number of the second lens.
Nevertheless, in the configuration of the three-unit zoom lens disclosed in Japanese Laid-Open Patent Publication No. 2006-10895, the image side positive power lens of the first lens unit has a low refractive index and still is a spherical lens. This causes a problem of insufficiency in the compensation of curvature of field.
Further, in the three-unit zoom lens disclosed in Japanese Laid-Open Patent Publication No. 2006-10895, when the focal length of the first lens unit is set up shorter for the purpose of size reduction, the diameter of the lens can be constructed relatively small. Nevertheless, when the first lens unit is constructed from two lenses, the optical power of the object side lens need be increased, and hence the thickness of the image side lens also need be increased for the purpose of compensation of chromatic aberration. This causes a problem of increase in the overall optical length at the time of non-use.
In the zoom lens of three-unit construction disclosed in Japanese Laid-Open Patent Publication No. 2004-144947, the optical power of the plastic lenses has a high ratio within the optical power of the entire second unit. Thus, when the optical power of the plastic lens is to be maintained low, the optical power of the entire second unit also becomes low. Thus, for the purpose of view angle enhancement, the amount of movement of the second unit during variable magnification need be increased. This causes difficulty in thickness reduction of the entire zoom lens.
In the zoom lens of three-unit construction disclosed in Japanese Laid-Open Patent Publication No. 2002-372667, since a plastic lens is employed, weight reduction and manufacturing error reduction are achieved to some extent. Nevertheless, these weight reduction and manufacturing error reduction are not satisfactorily achieved simultaneously to improvement in the optical performance and thickness reduction of the entire zoom lens.
In the variable magnification optical system of three-unit construction disclosed in Japanese Laid-Open Patent Publication No. 2006-11096, importance is imparted to the compactness. Thus, a problem of insufficient compensation of curvature of field is present. Further, the optical power of the second lens unit is low, and hence a zoom ratio of 3 or greater is difficult to be achieved.
In the zoom lens of three-unit construction disclosed in Japanese Laid-Open Patent Publication No. 2006-194974, compensation of magnification chromatic aberration is performed mainly by adjusting the refractive indices and the Abbe numbers of the lenses in the first lens unit as well as by adopting a cemented lens for the second lens unit. Nevertheless, compensation of various kinds of aberration is insufficient for the purpose of achieving a zoom ratio of 3 or greater. Further, view angle enhancement is also difficult to be achieved.